Comment-based syntax typically uses XML-like comments or special preprocessed comment-like elements to either annotate parts of a Web page (see StructureAnnotation) or to introduce program code into the document (see PythonInWebPage).

See PythonInWebPage for an example using JSP-like syntax. Here is a StructureAnnotation version using HTML-like syntax:

<table>
  <!-- loop:item in items -->
    <tr>
      <th>Name</th>
      <td><!-- expr:item.name --></td>
    </tr>
  <!-- loop:end -->
</table>

One potential advantage of comment-based syntax is that HTML-aware tools can ignore the comments, should the syntax have been chosen correctly.